1. Field of the Invention
The present invention relates to optical sources of coherent radiation and in particular to a new class of short wavelength lasers and optical parametric oscillators based of the frequency conversion or doubling inside a multimode waveguide. Such short wavelength microlasers have many important industrial, military and medical applications.
2. Information Disclosure Statement
Short wavelength lasers (green-blue lasers) and tunable frequency lasers have found many important scientific, industrial, military and medical applications. Some of this devices are commercially available and miniaturization of these lasers is required for many practical applications. Unfortunately, it is difficult to find appropriate active materials required for short radiation wavelengths. For example, in order to design short wavelength semiconductor lasers new semiconductor materials having a large energy gap should be developed. One of the solution of the problem is based on employing the effect of frequency conversion or doubling in appropriate nonlinear materials to lower the wavelength of radiation. A green wavelength laser based on this principle is now commercially available. Scaling of its output power, however, has some major restrictions. The design of the lasers based on frequency conversion or doubling requires both a means for high amplification of the fundamental frequency beam in an active laser material and a means for efficient frequency conversion in a nonlinear material. To enhance the efficiency of frequency conversion or doubling in a given nonlinear material one should increase the power of the fundamental frequency beam and the length of the nonlinear medium. Enhancing amplification of the beam in a given active laser material also requires increasing the length of the active laser medium. Moreover, conditions to effectively pump the active laser media should be provided. All these problems can be solved with lasers based on multimode waveguide structures. Relatively large cross section and large length of the multimode waveguide provides conditions to efficient pump the active material while the waveguiding properties of the nonlinear material provide strong field confinement over a large distance, as required for efficient frequency conversion.
Unfortunately, using standard multimode nonlinear waveguides can not provide efficient frequency conversion since the mode fields corresponding to the fundamental frequency and the converted frequency (second harmonic) do not overlap sufficiently. Moreover, it is not easy to realize phase matching in such multimode nonlinear waveguides as required for efficient frequency conversion. To provide this phase matching one should fabricate, for example, special periodicity or gratings inside the waveguide. In addition, the work with standard multimode waveguides requires the complicated processing of a mode speckle pattern at the waveguide output. The difficulty of coupling of the output beam into a single-mode information transmission fiber network also remain problems to be solved. These problems have severely restricted deteriorated the use of multimode waveguides in their applications.
It is the aim of the present invention to describe novel types of short wavelength lasers and optical parametric oscillators based on a new family of compound multimode optical waveguides which are designed to exploit specific useful properties of their higher order modes while providing good compatibility with single-mode optical fibers. A special compound structure of the compound multimode waveguides provides a unique possibility to achieve wavelength selection, efficient overlap of the modal fields corresponding to different frequencies and good phase matching. This compound waveguide structure supports modes having sharp peaks of their fields in the nonlinear waveguide region that also enhances the frequency conversion efficiency. Moreover, existence of this peak simplifies selection of this mode and efficient coupling of its radiation into a single-mode fiber networks. The combination of all these factors makes the system really unique.